Millions of trees, many of them full-grown Douglas firs, were flattened by blast over an area of nearly 600 square kilometers. Most were simply uprooted and stripped as bare as telegraph poles. Some, partially sheltered behind ridges, were snapped like match sticks. Initially, the blast had a velocity of 90 - 100 meters per second. It probably accelerated to supersonic velocities (greater than 300 meters per second) as it expanded in the vent area. Since it was travelling so much faster than the avalanche, the blast overtook the avalanche before the latter had travelled more than a kilometer or two. Since wind speeds of as little as 20 meters per second (80 kilometers per hour) can blow trees down, it is scarcely surprising that the blast levelled the forest so comprehensively. Note how the trees in the photo are pointing radially away from the volcano. They were all stripped of their foliage, so that only bare trunks remain. Note also how little volcanic material was deposited. The blast was a rapidly moving, low density cloud of dust and gas, from which denser material sedimented to form a deposit a few centimeters thick; at the margin of the blast zone, it density became low enough that it lofted away from the ground. (Section 4.4.2).

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